Yellow band two speed hubs were not used on the 20" wheel sized models. The Yellow band (with brake shoes) and the red band (with brake disks) were both under drive hubs. High gear was direct drive, and low gear was under driven.
The Blue band hub was used on the 20" wheel models because it was a "over driven" hub. Low gear was direct drive, and high gear was over driven making it much nicer geared on a small 20" wheel. The blue band hub received the brake show upgrade like the yellow band hub.
Very few parts interchange between the three hubs. The Indexing spring (the most common part to fail) works in all three hubs.
Almost all 20" wheels (a few exceptions on BMX) were only 28 hole wheel. I do not believe I have ever seen a 36 hole Blue Band hub.
John
Always great posts John, I think I can add a little to it.
I thought the reason for the OD hub was not only for the smaller wheel but the smaller lucky 7 chainring gearing. Funny thing about the blue band OD. The gearing isn't that much different then the yellow or red because it has a 20 tooth sprocket. If they could have fitted a 17 tooth sprocket (18 standard, 19 optional) on the yellow ratios would be about the same. But it is an overdrive.
A few parts interchange between all three hubs. There are 4 parts that are different, but every part interchanges the blue and yellow.
1) paint, hubs are the same just different paint.
2) Brake arm script, one says Overdrive the other mdl2
3) Sprocket, AB-402 20t for the blue band. AB-318 18t for the yellow.
4) Driving screw, AB-418 for the blue. AB-302 for the yellow.
If you swap numbers 3 and 4 you can make a blue a yellow or a yellow a blue. But you have to swap both.
I think the OD offered on the mini twin was a 36 hole and there was a 26 inch lightweight bike offered with a blue band, also 36 hole.
PN AB-401, 36 hole hub shell .080 gauge.
PN 480840, complete hub 36 hole .080 gauge.